Telephone system



led Nov.

TELEPHONE SYSTEK O. D. GRANDSTAFF INVENTOR.

. G RANDS TAF F ATTORNEY.

Maly 23, 1939.

...ov o 13009 a luana:

OT HO Patented May 23, 1939 UNITED STATES PATENT .-.OFFICE TELEPHONE SYSTEM Application November 14, 1936, Serial No. 110,864

13 Claims.

The present invention relates to telephone systems in general, but is concerned more particularly with telephone systems wherein automatic switches employed in the setting up of connections are controlled by alternating current pulses transmited over the talking conductors of intere'xchange toll or long distance lines. The object of the invention is the production of a new and improved auxiliary unit known as a tone unit, one of which tone units isassociated with each end of a two-way alternating-current-dialling toll line, being interconnected with a two-Way impulse repeater.

The tone unit disclosed herein isa direct improvement on the tone unit illustrated in Fig. 4 v of the drawings in Patent 2,040,637, granted May 12, 1936 to John Wicks. The improved tone unit may be used to replace the tone unit shown in Fig. 4 of the Wicks patent, as the leads incoming 20 to the tone unit disclosed herein are similar in function and number to the leads interconnecting the tone unit in the Wicks patent with the associated repeater. The new tone unit, however, was provided particularly for cooperation 25 with new and improved impulse repeaters of the type shown in the application of John Wicks, Serial Number 109,671, filed November 9, 1936. In this Wicks application, the present tone unit. is fully illustrated and is rather completely de- 30 scribed in order to bring out the novel features ofthe system disclosed in the said Wicks application.

'I'he tone unit disclosed herein contains features of improvement over the corresponding 35 lequipment in the Wicks patent referred to, in-- cluding the following features:

1) In order to improve the signal reception, in addition to the main tuner which is resonant at the signal carrier frequency, a second tuner is 40 employed which is resonant at a frequency lying below the signal carrier frequency and also at a frequency lying above tne signal carrier frequency.. This additional double-peak tuner is associated through suitable circuit connections with the apparatus so that current flowing therethrough opposes the action of the current flowing through the main tuner. One desirable result ofy this arrangement is that sharp frequency cutl 66 effect of the current from the double peak tuner is sufficient to limit the response and thereby lessen the tendency to overload the succeeding apparatus. The detecting or rectifying apparatus which succeeds the tuners is so arranged that the potential built up at the double-peak tuner 5 must overcome a substantial adverse bias before detection or rectification of current therefrom can occur, whereby the' opposing effect from the double-peak tuner is entirely absent in the event that weak signals are being received, as it is undesirable that la. Weak signal be diminished in any way by the opposing effect of the doublepeak tuner.

(2) The general method of detecting utilized in the Wicks patent referred to has been retained in the present disclosure, but has been improved Y to enable a wider signal rangeto be accommodated. The signal rectification is carried out through a tube of the diode type, and the rectified potential is supplied to the grid element of an amplifier tube so related to the diode tube that the plate current controlled by the rectified signals varies directly with the signal strength instead of varying inversely as in the prior structure. By this arrangement, the responding relay is normally restored and operates responsive to the rise of current when a signal is received. Tests reveal that this arrangement gives greatly superior results when the wide range of signal strength metvwith in practice is taken into account.

(3) In order to secure a more reliable operation of the incoming signal relay, which responds only when the received carrier current is modulated at the predetermined modulating frequency, the output of the impulse amplifier is further amplified by a. signal amplifier before being passed to the circuit of the incoming signal relay. By this arrangement a strong response to the incoming signal relay is assured over the maximum range of input signal strength.

(4) An improved method of modulating the transmitted carrier frequency has been employed. Whereas the prior arrangement utilized a supply of plate current taken directly from the Source of modulating alternating current, the present arrangement supplies modulating potential to a grid element of the amplifier tube lying between the screen grid and the anode element, while the carrier frequency is impressed in the usual way through the application of a potential at carrier frequency to the control grid of the tube.

By choosing an implifier tube having the desired characteristics, the modulated carrier current flows at substantially full strength throughtone unit is shown diagrammatically therein,4

together with conductors leading through cable |200 assumed to extend to an associated impulse repeater as disclosed in the pending Wicks application hereinbefore referred to.

Since it is undesirable in telephone practice that. any of the trunk or line equipment be disabled upon the failure of the commercial `power supply, the tone unit here illustrated has been arranged as far as possible to be independent of a temporary power failure. The heaters of the vacuum tubes are supplied with current from the 48-volt exchange battery, and the plate and screen-grid current is obtained from a separate 13o-volt storage battery. The 1000-cycle potential `normally impressedon the common lead |242 is preferably obtained from a 1000-cycle generator operated from the 48-volt exchange battery, while the GO-cycle, 40-volt modulation potential impressed on the common lead 124| is preferably obtained normally from the 60-cycle source of commercial current, with automatic switching arrangements effective in the event of a failure of the commercial source of power to switch this lead over to a stand-by 60-cycle generator which is automatically started into operation drawing power from the exchange battery. By the grid modulation arrangement disclosed, only a negligiblev amount of 60-cycle power need be generated by a local generator when the commercial power fails.

The signals received over the associated toll line come into the tone unit through receiving conductors H62 of the cable |200, while the transmitted signals go out over sending conductors ||6I of the cable |200 to the conductors of A the toll line.

Direction-transfer relay |202 is normally en- .ergized over direction-transfer conductor of cable |200, through contacts of key |234. With direction-transfer relay |202 in` its normally energized condition, the tone unit is conditioned for the reception of signals. 'I'his relay is restored to adapt the tone unit for transmission. Modulation-cut-oif relay |20I is controlled over modulation-cut-off conductor I|61 ofthe cable |200 to substitute ground potential for the 60- cycle modulation conductor |'24I when unmodu lated signals are to be transmitted. I

Pulse-cut-in relay |204 is controlled over pulsecut-in lead H66 of the cable 200 to substitute incoming-impulse relay |203 for plate-impedance coil |201 when the tone unit is to be conditioned for the reception of dial pulses. These dial pulses are repeated over the dial-in conductor I |164 of the cable |200 for an incoming repeating relay oi' the associated impulse repeater.

The incoming-signal relay |205 transmits its signals over the signal-in conductor H63 of the cable |200 to a signal-repeating relay of the associated impulse repeater.

It may be pointed out that receiving leads I |62 of the cable |200 correspond to receiving leads 391 and 392, Fig. 4 of the previously mentioned Patent 2,040,637, granted to Wicks; sending leads I|6I correspond to sending leads 393 and 394 of the Wicks patent; modulation-cut-off relay I20| and modulation-cut-oif conductor H61 correspond respectively to relay 324 and lead 395;

direction-transfer relay |202 and the associated Vcable conductor H65 correspond respectively to relay 325 and conductor 396 in the Wicks patent; pulse-cut-in relay |204 and the associated cable conductor H66 correspond respectively to relay 401 and the associated lead in the Wicks patent; and the signal-in and dial-in leads H63 and |I64 correspond respectively to the leads in the said Wicks patent which are controlled by relays 404 and 402, respectively.

The tone unit includes vacuum tubes I2I I, I2I2, |2I3, and |2|4. The tubes |2|I and |2|3 are of the type known as 6C6, each being provided with a heater of a nominal rating of 6.3 volts, at which voltage it draws .3 ampere. Tube I2| 2 is a double-diode tube arranged to function as a differential detector of incoming signals. 'Ihis tube is of the type known as 6H6, being provided with a two-section heating which requires a potential difference of 6.3 volts to pass .3 ampere throughit.

The signal amplifier |2|4 is a tube of the type known as 43, having a 25-volt heater which also is rated to draw .3`ampere. The heaters of these tubes are all connected in series, as shown, and they are normally drawing current in the circuit 'with the 4ohm resistor |225 through contacts of the normally-actuated key |231, the connection with the negative pole of the 48-pole exchange battery being made through the 10-ohm test relay |033. 'This test relay is normally actuated, but is permitted to restore and close a circuit for the signal lamp |236 inthe event that any one of the tubes is removed from its socket or develops a burned out or broken heater element. The potential-drop along the heater circuit from the negative pole oi the 48volt battery is used at various points for grid-biasing purposes, as will be pointed out.

Tube I2|I is used as a two-way amplier, being used as an outgoing amplifier and modulator in the transmission of signals to the associated toll line. This tube is provided with a cathode as indicated, adjacent which is shown the control grid. Adjacent the control grid is the screen grid which is supplied with positive potential from the 13G-volt source of plate current through the 50,000-ohm resistor |221, being by-passed to groundl by the .1 mf. ,condenser |220. Between the control grid and the plate, there s shown the suppressor grid (used as a modulation grid) which is connected either to ground potential or to the (iO-cycle, 40-volt lead I24I, depending upon the actuated or restored condition of relays I20I and |202.

For the reception of signals, the primary winding of the 1 to 30 ratio step-up transformer |224 is connected to the receiving conductors I|62 through the two illustrated .1 mf. condensers. The secondary winding of this transformer is normally connected to control the potential applied to the control grid of tube I 2| I as a negative biasing potential of 1.2 volts being applied to the control grid of tube I2II by virtue of the 1.2 volt drop across the 4-ohm resistor |225.

With direction-transfer relay |202 in operated position, as it normally is, the secondary winding of the30 to 1 ratio step-down transformer I2I1 is connected across the leads |243 and |244. 'I'he inductance coil |223 is connected across the leads |243 and I 244 in series with the .04 mf. condenser |222, which condenser is shunted by the indicated trimmer condenser of such capacity as to render the couple series-resonant at 1000 cycles,

whereby a relatively high potential is placed on the righthand anode of the differential detector |2|2 by the voltage drop across coi-l |223 when a 1000-cycle current isbeing received and amplified by tube |2|I.

In order to enable the system to respond to impulses of 1000 cycles, plus or minus an amount equal to about sixty cycles so as to allow for the reception of GO-cycle-modulated, 1000 cycle pulses. ,and to'cut off sharply when frequencies higher or lower than the allowable band are received, the inductance coils |2|3 and |22|, with their associated tuning condensers are arranged to form what may be termed a double-peak tuner. This tuner peaks atv 900 cycles and at 1100 cycles. 'I'he 900- and 1100-cycle peaks are in addition to the peak produced by the inductance coil ||23 and its associated condensers, as previously pointed out. Coil |22|, in series with the tuning condenser |220 (assisted bythe associated trimmer condenser), is resonant at ||00 cycles, provided substantially no current is permitted to flow through the shunt condenser |2|0. The last named condenser has a capacity oi' about .01 mf. Current of 1100 cycles is substantially prevented from flowing through condenser |2|0 by the shunt-resonant effect of coil |2 I9 and the associated shunt condensers, as this system is shunt-resonant at 1100 cycles. When current is received at some other frequency, however, the shunt-resonant condition is not present at coil |2|9 and the associated shunt tuning condensers, permitting current to pass through condenser |2|0 by way of coil |2|9 and the associated shunt condensers. The value of condensers |2|0 is so chosen that it (connected in multiple with tuning condenser |220 through the system composed of coil |2|9 and its associated condensers) causes the coil |22| to have a resonant peak at 900 cycles, in addition to the previously described one at 1100 cycles.

'I'he way in which the triple-peak tuning ar- -rangement operates to control the impulse arnpliiier |2|3 by way of the differential detector |2|2 will now be pointed out:

It is to be noted that the control grid of the impulse amplifier |2|3 is normally provided with a negative grid bias of 20.1 volts by virtuey of its connections to the heater circuit, which amount represents the drop across the heaters of tubes |2|3, |2|2,and |2|I, in addition to the drop across the 4-ohm resistor |225. This grid biasing potential is supplied to the control grid of tube |2|3 through the center-tapped .-megohm resistor comprisingA sections |229 and |230. Each section of this resistor is by-passed by .1 mf. storage condenser, as shown. By virtue of the connection of conductor |244 to the center tap of the grid resistor associated with impulse amplifier |2|3, and the connection of the lower terminal of this resistor to the heater circuit 'at a point of negative 2.0.1 volts, a negative potential of 20.1 volts is normally maintained on the two plates of the differential .detector |2|2 through coils |22| and |223. The cathode associated with the plate connected to the 1000-cycle coil |223 is also supplied with a biasing potential of negative 20.1 volts, whereby there is normally no potential difference between the cathode and anodevof tube |2|2 with respect to coil |223.

The cathode associated with the double-peak coil |22| (resonant at 900 cycles at 1100 cycles) is connected to the heater circuit at a point of negative 1.2 volts potential, whereby the plate connected to coil |22| isnormally 18.9 volts more negative than the associated cathode, by virtue of which fact no current passes through the lefthand portion of the differential detector until the rather substantial adverse bias 'of 18.9 volts is overcome.

During the reception of a signal, the building up of a 1000-cycle potential ,across coil |223 causs current to' flow through the associated cathode and anode of tube |2 I2 in series with the section |229 of the grid resistor. Thus halfwave-rectified current passes through the upper section of |229 of the resistor and renders the upper terminal of this resistor section more positive than the lower terminal, thereby offsetting to this extent the negative potential normally applied to the control grid of impulse amplifier |2|3. On the other hand, when the signal contains a suflicient component at 900 cycles or 1100 cycles to cause a sufficient potential to be built up across coil |22| to overcome the 18.9 volts bias, current flows in the left-hand section of the differential detector I 2|2. This half-wave-rectifled current passes through the lower section |230 of the grid resistor associated with the tube.

|2|3. causing the mid-point of this resistor to become more negative than the lower terminal thereof by virtue of the potential drop through section |230. Therefore, when a signal other than a wanted signal is received and which falls in the neighborhood of 900 cycles or 1100 cycles, the current flowing through coil |22| and the associated section of the differential detector and obtained through the section |230 of the grid resistor associated with tube |2|3, tends to render the grid of tube |2|3 more negative than it normally is, thereby preventing a response at tube |2|3 to the signal, even though sufficient current may flow through coil |223 to otherwise secure a response.

It is to be noted that pulse-cut-in relay |204 is provided to be controlled over pulse-cut-in conductor H06. When the tone unit is to be used to receive dial impulses from a distant exchange, relay |204 is operated to substitute incoming-impulse relay |203 for retard coil |201 in the plate circuit of impulse amplifier |2|3. The previously mentioned grid bias normally existing on the control grid of tube |2|34 is sufficient to block the flow of current from the volt source of plate current. However, when a signal is received which causes the potential on the control grid of tube |2|3 to be brought down to a negative value of about sixteen volts, current starts to flow through the tube, it being noted that the screen grid and suppressor grid of this tube are lconnected to the plate to secure a. triode of the desired characteristics. As the potential of the control grid is rendered still less negative, relay |203 operates when about 4.5 milliamperes of current are flowing in the plate circuit, about 9 milliamperes being preferable for operating the relay reliably during the reception of dial pulses.

A fairly constant response of incoming impulse relay |203 over a. wide range of input voltage is inherent in the disclosed arrangement because of the tendency of the grid-cathode circuit of tube |2|3 to draw a grid current when the input signal voltage is sufficiently high to overcome the normal biasing voltage of negative 20.1 volts. This characteristic of the tube imposes a rather definite limit on the swing of the grid toward a positive-potential value, permitting a fast recovery of' the negative blocking potential betweeny signal pulses. Keeping in mind that the coil |223 and the associated tuning condensers |222 com- By arranging that the response curve of the coils |22| and |223 intermingle (coil |22| peaks at 900 and'at 1100 but still builds up an appreciable voltage at 1000 cycles) the voltage built up across the double-peak coil |22| at the 1000-cycle signal frequency and over the upper portion of the signal-strength range is sufficient to reduce" considerably the positive grid-swing tendency, thereby further tending to secure a uniform response over a wide range of input voltage. The weak signals, however, are not handicapped by the opposing effect of coil |22|, as the adverse bias is not then overcome.

The signal amplifier I2 |4 is provided to amplify the 60-cycle modulations received at the plate of impulse amplier |2|3. The control grid of the tube |2|4 is provided with a normal `negative biasing potential of 20.1 volts through the 100,000-ohm grid resistor |232. At this biasing voltage, substantial plate current flows through the plate circuit of the tube through the plate impedance coil |208 from the 13D-volt source of plate current, the screen grid of this tube being connected directly to the source of plate current while the suppressor grid is connected .directly to the cathode to provide the usual form of pentode operation. The .1 mf. coupling condenser |23| serves to couple the plate of tube |2|3 with the control grid of tube |2|4, to transmit the 60-cycle signal impulses to the signal amplifier.

After amplification by tube 2| 4, the signal impulses pass to the incoming-signal relay |205 by Way of the .3 mf. condenser |234. Condenser |234 and the winding of relay |205 are so chosen that the circuit containing them` is series resonant at about 60 cycles, whereby the incoming signal responds vigorously to 60-cycle current, but does not respond when the 1000-cycle dial impulses, following each other at about ten to fourteen impulses per second, are received.

'I'he .006 mf. condenser |235 is provided to bypass amplified, half-wave-rectified 1000-cycle carrier current from relay |205, as it is found in practice that otherwise amplified 1000-cycle potential is sufficiently high to cause undesired brush discharges. l v

With the tone unit in normal position direction-transfer relay |202 is operated over direction-transfer:conductor |65. Also, pulse-cutin relay |204jis in 'restored condition, whereby incoming impulse relay |203 is out of circuit and the retard coil |201 is in circuit. This is the condition in which the'tone unit receives a 60- cycle-modulated seizing pulse when the associated toll line is taken for use at the distant end. This pulse comes intothe tone unit through the receiving conductors |62 and passes through the transformer |224 to the grid-cathode circuit of the two-way amplifier tube |2| The signal is .amplified and passes through condenser |226 to the' primary winding of transformer |2|'|. From the secondary winding of this transformer, the

signal passes through contacts of the normally operated direction-transfer relay |202 to the leads |243 and |244 associated with the tuning coils.

` This signal then passes through the differential through its winding in series with the condenser |234, the condenser |234 and the winding of the relay being substantially resonant to the 60-cycle modulating frequency. Upon responding, incoming signal relay |205 extends ground potential for the duration of the signal `over the signal-in conductor |63:v Relay |205 restores and again removes ground potential from conductor ||63 when the seizing pulse is terminated.

When the associated repeater responds to the seizing pulse and actuates the pulse-cut-in relay |204 over conduc-tor ||66, incoming pulse relay |203 is substituted for plate impedance coil |201. As a result, when the dial pulses of unmodulated 1000-cycle current arrive, these impulses pass through as above explained to the incoming signal relay |205, but this relay does not respond at this time because the frequency of the pulses does not correspond to the frequency to which the coil of relay |205 and the condenser |234 are resonant. Relay. |203 is operated momentarilyupon the receipt of each of these dial pulses, momentarily grounding the dial-in conductor H64 each time is operates.

When the pulse-cut-in relay |204 is subsequently restored by the associated repeater, incoming-impulse relay 203 is removed from circuit and plate-impedance coil |201 is reintroduced, preventing further response of relay |203, while permitting incoming signal relay |205 to respond to any further received current at carrier frequency which is modulated at the predetermined 60-cycle rate.

When the tone unit is-to be used to transmit a seizing pulse over the associated toll line. direction-transfer relay |202 is restored to transfer the grid of the tube'|2|| from the upper terminal of coil |224 to the 1000-cycle control lead |242. When this occurs, a 1000-cycle variation is caused to beproduced in the plate current normally passing through plate impedance coil |206. At the same time, the suppressor grid of the tube |21||, the grid lying just within the plate or anode element of the tube, is extended through contacts of the non-operated relay |20| 'to the l0-volt, (iO-cycle modulation lead |24|.

ary winding of transformer |2|'|, with relay |202` restored, is connected to the sending conductors -||6|, while the mid-point of this winding is grounded for purposes having to do With line stabilization. The sending conductors ||6| are suitably connected with the toll line through the associated impulse repeater to permit the 60- cycle modulated 1000-cycle current to pass over the toll line.

At the'end of the transmission of the seizing pulse, direction-transfer relay |202 may be permitted to reoperate to return the tone to the normally existing receiving condition.

.In order to prepare for the transmission of series of unmodulated 100G-cycle pulse, modulation'ut-oil' relayr |20| may be operated over modulation cut-oil.' conductor H61 to thereby connect the associated back contact of the concerned armature of relay |202 to ground instead of to modulation conductor |24|. In this way,A when direction-transfer relay |202 is next restored, unmodulated 1000-cycle current ows from the tube |2|| through coupling condenser |220 to the transformer |2|'|, and passes throughr this transformer, through contacts of the restored relay |202, to the sending leads HSI. This 1000- cycle current is suitably impressed through pulsing contacts of the impulse repeater on the conductors of the toll line. l

The modulation-cut-oif relay |20| may be again restored after the dial pulses have been transmitted to permit the transmission of further 60-cycle-modulated, 100G-cycle current, the varying control of the relay |20| and |202 being as ,explained in the Wicks patent and in the Wicks application referred to hereinbefore.

In the event that it is desirable during repair or test of the unit to open the heater circuit, the key |231 may be restored for this purpose. Additional contacts of key |231 open a point in the circuit of lamp I236to avoid lighting this troublesignal lamp upon the restoration of test relay |203, which occurs when the heater circuit is opened.

In Lthe event that it is desired to test the transmission level of the tone unit, the key |234 may be actuated and a test plug associated with the test equipment may be inserted into the jack |2|5, connected to the sending leads ||6|. Upon the actuation of key |234 direction-transfer relay |202 is restored and modulatlon-cut-off relay |20| is operated. In the event that the transmission level is not satisfactory, it may be adjusted by moving the slide arm of the adjustable shunt resistor |2I6.

When the test and adjustment have been accomplished, the plug may be removed from the jack I2|5 and the key |234 may be restored.

` Upon the restoration of this key, relay |202 is permitted to reoperate and relay |20| is permitted to restore.

The connection between thecontrol grid of tube i 'further' reduces the inter-contact capacity which may otherwise be sufficient to impress a slight local 1000-cycle modulation on the control grid with relay |202 operated. This tendency is rendered practically non-existent by the grounded front contact of the intermediate armature under discussion.

i What is claimed is:

1. In a signalling system wherein received modulated carrier current is rectified to produce direct current, a responding relay arranged to respond to impulses of rectified carrier current to repeat corresponding signal pulses into a local circuit, a circuit resonant at the modulation frequency of the received carrier current, a` signal relay responsive tothe flow of resonant current in said resonant circuit, an amplifying device'for `supplying power to said resonant circuit, and cir- 2. In combination, a source of alternating current signals, a conductor on which said signals are to be impressed, a first rectifier associated with said source for impressing rectified signal potentials on said conductor, a second rectifier interconnected with said conductor and effective to draw curr-ent from said conductor to reduce the potential thereon, means for impressing a biasing potential opposing the drawing of current from said conductor except when the rectified potential thereon is sufficient to overcome the biasing potential, a responding device, and means for causing said device to respond to signals according to the potential existing on said conductor.

3. In combination, an electron discharge device including input and output electrodes, an output circuit coupled to said output electrodes,

va tuned circuit having a frequency response characteristic which peaks at a predetermined frequency, a tuned circuit having a frequency response characteristic which peaks at frequencies respectively above and below said predetermined frequency, said tuned circuits being adapted to be coupled to a source of signal currents, means for impressing a direct voltage between said input electrodes which is controlled in accordance with the signal voltage across one of said tuned circuits, and means for impressing a direct voltage between said input electrodes which is controlled in accordance with the signal voltage across the other of said tuned circuits and which is in opposition to said firstnamed direct voltage, whereby the current in said output circuit is controlled in accordance with the difference in magnitudes of the signal voltages across said tuned circuits.

4. In combination, an electron discharge device including input and output electrodes, a tuned circuit having a response characteristic which peaks at a predetermined frequency, a tuned circuit having a response characteristic which peaks at frequencies substantially equally spaced respectively above and below said predetermined frequency, the peak frequencies of said last-named tuned circuit being displaced substantially equally on either side of said predetermined frequency, means for impressing a direct voltage between said input electrodes which is controlled in accordance with the signal voltage across one of said tuned circuits, and means for impressing a dir ect voltage between said input electrodes which is controlled in accordance with the signal voltage across the other of said tuned circuits and which is in opposition to said firstnamed direct voltage, whereby the current in said output circuit is controlled in accordance with the difference in magnitudes of the signal voltages across said tuned circuits.

5. In combination, an electron discharge device including input and output electrodes, an output circuit coupled to said output electrodes, a tuned circuit having a frequency response characteristic which peaks at a predetermined frequency, a tuned circuit having a frequency response characteristic which peaks at frequencies respectively above and below said predetermined frequency, said tuned circuits being adapted to be coupled to a source of signal currents, and a network including rectifying means interconnecting said tuned circuits and said input electrodes, said network being so connected and arranged that a direct voltage, variable in accordance with the signal voltage across said iirstnamed tuned circuit, is impressed on said input electrodes, and a second direct voltage, opposed to said first direct voltage and variable in accordance with the voltage across said secondnamed tuned circuit,is impressed on said input electrodes, whereby the current in said output circuit is controlled in accordance with the difference in magnitudes of the signal voltages across said tuned circuits.

6. In combination, an electron discharge device including input and output electrodes, an output circuit coupled to said output electrodes, a tuned circuit having a frequency response characteristic which peaks at a predetermined frequency, a tuned circuit having a frequency response characteristic which peaks at frequencies respectively above and below said predetermined frequency, said tuned circuits being adapted to be coupled to a source of signal currents, means normally biasing said device beyond cut 0E, means for impressing va direct voltage between said input electrodes which varies in accordance with the signal voltage across said first-named tuned circuit, whereby a current in said output circuit is only produced when the signal voltage across said first-named tuned circuit attains a substantial value, and means for impressing a direct voltage between said input electrodes which varies in accordance with the signal voltage across said second-named tuned circuit and which is in opposition to said first-named direct voltage whereby the current in said output circuit is controlled in accordance with the difference in magnitudes of the signal voltages across said tuned circuits.

' 7. In combination, an electron discharge device including input and output electrodes, an output circuit coupled to said output electrodes, a tuned circuit having a frequency response characteristic which peaks at a .predetermined frequency, a tuned circuit having a frequency response characteristicv which peaks at frequencies respectively above and below said predetermined frequency, said tuned circuits being adapted to be coupled to a source of signal currents, means normally biasing said device beyond cut off, means for impressing a direct voltage between said input electrodes which varies in accordance with the signal voltage across said first-named tuned circuit, whereby a current in said output circuit is only produced when the signal voltage across said first-named tuned circuit attains a substantial value, and means for impressing a,

cuit at a negligible value until the signal voltage across said first-named Vtuned circuit attains said substantial value. y y

8. In combination, an electron discharge device including input and output electrodes, an output circuit coupled to said output electrodes, a biasing network connected between said input electrodes, a tuned circuit having a frequency response characteristic which peaks at a predetermined frequency, a tuned circuit having a frequency response characteristic which peaks at frequencies respectively above and below said predetermined frequency, said tuned circuits being adapted to be coupled to a source of signal currents, a rectifier coupled to said first-named tuned circuit and connected to impress its rectified output lvoltage across at least a portion of said biasing network, and a second rectifier coupled to said second-named tuned circuitl and connected to impress its rectified output voltage across at least a portion of said biasing network in opposition to said first-named rectified voltage, whereby the current in said output circuit is controlled in accordance with the difference in magnitudes of the signal voltages across said tuned circuits.

9. In combination, an .'electron discharge device including input and output electrodes, an output circuit coupled to said output electrodes, Aa biasing network connected between said input electrodes, a tuned circuit having a frequency response characteristic which peaks at la predetermined frequency, a tuned circuit having a .frequency response characteristic which peaks at frequencies respectively above and below said predetermined frequency, said tuned circuits being adapted to be ycoupled to a source of signal currents, a source of biasing voltage connected to said.biasing network and normally effective to bias said-device beyond cut off, a rectifier coupled to said first-named tuned circuit and connected to impress its rectied output voltage across at least a portion of saidnetwork in opposition to said biasing voltage, and a second rectifier coupled to said second-named tuned circuit'and connected to imp ress its rectified output voltage across at least a portion of said network in the correct phase to boost said biasing voltage, whereby a current in said output circuit is only produced when the signal voltage across said first-named tuned circuit attains a substantial value and the current in said output circuit is controlled in accordance with the difference in magnitudes of the signal voltages cross said tuned circuits.

10. A signal receiving device comprising, in combination, an inductance element, means comprising a capacitor for tuning said element to resonance at a predetermined frequency, means comprising a second capacitor for` tuning said element to resonance at a second frequency, and means comprising a circuit resonant at said predetermined frequency for preventing said second-named capacitor from substantially affecting the response of said device at said predetermined frequency.

11. A signal receiving device comprising, in combination, an inductance element, a capacitor connected in series with said inductance element to tune said element to resonance at a predetermined frequency, a second .capacitor connected in series with said element to tune said element to resonance at a second frequency which is substantially lower than said predetermined frequency, and means comprising a parallel resonant circuit tuned to said predetermined frequency and connected between said inductance element and said second-named capacitor for preventing said second-named capacitor from substantially affecting the response of said device at said predetermined frequency.

l2. A signal receiving device having input terminals and comprising, in combination, a pair of reactance elements having opposite frequencycurrent characteristics connected in series between said terminals, said elements having their reactance values so proportioned that the circuit formed thereby is resonant at a predeter- `mined frequency, and a network shunting one of said elements, said network comprising a reactance element of the same type as said one element and a parallel resonant circuit connected in series, the reactance valu of the elements of said network being so proportioned relative to each other that the series impedance thereof is substantially innlteat said predetermined frequency and that the combined circuit formed by the other of said pair of elements and said network is resonant at a second frequency different from said predetermined frequency.

13. A signal receiving device having input ter- Y minals and comprising, in combination, a pair of reactance elements having opposite frequencycurrent characteristics connected in series between said terminals, said elements having their i'eactance values so proportioned that the circuit formed thereby is resonant at a predetermined frequency, a network shunting one of said elements, said network comprising a reactance element of the same type as said one element and a parallel resonant circuit connected in series, the reactance values of the elements of said network being so proportioned relative to each other that the series impedance thereof is substantially innite at said predetermined frequency and that the combined circuit formed by the other of said pair of elements and said network is resonant at a second frequency different from said predetermined frequency, and a second circuit connected between said terminals, said second circuit comprising a pair of reactance elements having opposite frequency-current characteristics connected in series and having their reactance values so proportioned that said second circuit is resonant at a frequency substantially intermediate said predetermined frequency and said second frequency.

OTHO D. GRANDSTAFF. 

